Sensors are being developed for use in roads, bridges, dams, buildings, towers, and vehicles that may provide many information, including displacement, strain, speed, acceleration, temperature, pressure, and force.
Temperature changes can effect components within a sensor, and one challenge has been that sensor output has varied with temperature of sensor components. The effect of temperature on inductive transducers limits their overall absolute accuracy. To reduce the temperature effect, inductive transducers, such as a differential variable reluctance transformer (DVRT), have been designed with a differential pair of coils. A DVRT includes a ferrite core that moves within two coils that are arranged in a bridge configuration. Inductance and impedance of the coils changes with displacement of the ferrite core within them, and the displacement of the core is accurately determined by the change in inductance or impedance of the coils. Since the output signal of a differential pair is the difference between the output of two coils, temperature changes that both coils experience equally are theoretically subtracted out. However, if one coil experiences a different temperature environment than the other coil, a signal proportional to the temperature gradient between the two coils will appear at the circuit output, significantly reducing absolute accuracy.
Commonly assigned U.S. Pat. No. 5,914,593 (“the '593 patent”), incorporated herein by reference, provides a circuit for a DVRT that compensates for a temperature gradient across a sensor. The '593 patent recognizes that a temperature gradient across the coils can provide a change in the resistance of the wire forming one of the coils more than it changes the resistance in the other coil, and this difference in resistance can change the differential impedance of the coils, essentially mimicking a change in position, resulting in an error in measured displacement. Situations where one coil may be hotter than the other coil are common in applications such as automotive. In the '593 patent a circuit is provided to adjust the output compensating for the difference in resistance in the two coils introduced by a temperature gradient or a temperature difference between the coils.
However, the present inventors found that not all temperature effects were compensated by the circuit of the '593 patent. Thus, a better system for acquiring data is needed that provides improved temperature compensation, and this solution is provided by the following invention.